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Abstract:

Method for multiple access to the radio resources in a mobile ad hoc
network comprising a plurality of communicating mobile terminals or
nodes, having known geographical coordinates, including constructing the
said network, allocating reception frequency bands to each of the said
active nodes of the said network, the said frequency bands being of
identical width and spaced apart by a given minimum guard band, spreading
code allocation performed by each of the said receiver active nodes of
the said network for each of the said neighbor transmitter nodes to which
it is directly linked by an active link according to an asynchronous CDMA
method, and power control performed by each of the said receiver nodes of
the said network for each of the said neighbor transmitter nodes to which
it is directly linked by an active link.

Claims:

1. A method for multiple access to the radio resources in a mobile ad hoc
network comprising a plurality of communicating mobile terminals or
nodes, having known geographical coordinates, the method comprising:
constructing the said network making it possible to generate a plurality
of active links between at least two active nodes of the said network, an
active link being composed of two pairs of channels, assigned
respectively to transmission and to reception, each comprising a
signaling sub-channel and a communication sub-channel, a link being
activated when the degree of relevance of the said link is greater than a
given constant, the degree of relevance of the said link being determined
at least as a function of criteria regarding distance between the said
nodes and/or of criteria regarding priority between the said nodes, each
of the said active nodes of the said network comprising a neighborhood
table which groups together the set of active nodes of the said network
to which it is directly linked by an active link; allocating reception
frequency bands to each of the said active nodes of the said network, the
said frequency bands being of identical width and spaced apart by a given
minimum guard band, the said allocating being performed in the following
manner: if the number of active nodes of the said network is less than or
equal to the number of frequency bands available, a different reception
frequency band is allocated to each of the said nodes; and if the number
of active nodes of the said network is strictly greater than the number
of frequency bands available, a reuse of the said frequency bands is
effected in such a way that the allocation is performed so as to minimize
the interferences between two active nodes which have the same reception
frequency band; performing spreading code allocation by each of the said
receiver active nodes of the said network for each of the said neighbor
transmitter nodes to which it is directly linked by an active link
according to an asynchronous CDMA method, the said neighbor transmitter
nodes communicating with the said receiver node in the reception
frequency band which has been allocated to it; and performing power
control by each of the said receiver nodes of the said network for each
of the said neighbor transmitter nodes to which it is directly linked by
an active link, the said power control step implementing a different
power regulation loop for each of the said active links emanating from
the said receiver node having the function of ensuring that all the
signals received by the said receiver node originating from the said
transmitter nodes are so with the same power level.

2. The multiple access method according to claim 1, wherein the same
reception frequency band is allocated to a plurality of distinct active
nodes if and only if the disturbance level induced by an active link of
one of the said nodes on an active link of another of the said nodes is
below a given threshold.

3. The multiple access method according to claim 1, wherein a change of
reception frequency band of a node of the said network is performed
during transmission and that this change is announced to the other nodes
of the said network with the aid of announcement messages containing at
least the instant of the change and the new frequency band.

4. The multiple access method according to claim 1, wherein each of the
said nodes keeps up to date a geographical database comprising its
geographical coordinates as well as that of the nodes of its neighborhood
table, the said geographical coordinates being provided by a mechanism
external to the said network such as a long-range omnidirectional
waveform, a tactical data link or a radar system.

5. The multiple access method according to claim 1, wherein each of the
said nodes keeps up to date a geographical database comprising its
geographical coordinates as well as that of the nodes of its neighborhood
table, the said base being constructed and maintained with the aid of
signaling messages dedicated for this purpose and transmitted between the
nodes of the said network.

6. The multiple access method according to claim 5, wherein access to the
radio resources of a new node entering the said network is carried out
with the aid of at least of the following steps: the said entering node
randomly selects a reception frequency band from among those available;
the said entering node transmits at least one network entry message in
each of the frequency bands available with a transmit power whose level
increases progressively until a response is obtained, the said entry
message containing at least the geographical position of the said
entering node, its speed, its acceleration and its reception frequency
band; on receipt of a network entry message, an addressee node updates at
least its geographical database and its neighborhood table and responds
through a response message transmitted in the reception frequency band
contained in the said network entry message with a transmit power
calculated at least as a function of the distance between the said
entering node and the said addressee node, the said response message
containing at least the geographical position of the said addressee node,
its speed, its acceleration and its reception frequency band; and on
receipt of a response message, the said node entering the network updates
at least its geographical database and its neighborhood table.

7. The multiple access method according to claim 6, wherein the
dispatching of the said network entry and response messages is done under
access contention, using a single spreading code known to all the nodes
of the network.

8. A communication system onboard a mobile node communicating within a
mobile ad hoc network, the system comprising at least one
three-dimensional geographical database and a processor configured to
implement the method for multiple access to radio resources according to
claim 1.

9. A mobile ad hoc network comprising a plurality of mobile nodes,
wherein the said nodes carry onboard a communication system for multiple
access to the radio resources within the said network according to claim
8.

10. The mobile ad hoc network according to claim 9, wherein the said
mobile nodes are aircraft.

Description:

[0001] The present invention relates to a method for multiple access to
the radio resources in a mobile ad hoc network, for example a
high-throughput low-latency network, as well as to the system
implementing this method.

[0002] It advantageously applies in an aeronautical context where the
nodes of the ad hoc network are aircraft but may be implemented for any
type of mobile ad hoc network. The expression ad hoc network is
understood to mean a network consisting of mobile elements, also called
nodes, which at one and the same time combine the transmitter, receiver
and relay functions. Each node of an ad hoc network communicates directly
with its neighbors without passing through an intermediate access point
which would have the function of relaying or/and routing the packets
transmitted as is the case in a so-called infrastructure mode network. An
ad hoc network offers a topology of meshed type. In the subsequent
description, any mobile terminal communicating through a wireless link
with other mobile terminals organized and structured within a meshed
network will be called a network node. A node is for example, but not
solely, an aircraft, or a pilotless plane which carries onboard a
telecommunications system implementing the radio resources access method
according to the invention.

[0003] By way of example, an infrastructure-mode network is a mobile
telephone network of UMTS ("Universal Mobile Telecommunications System")
type. This type of network consists on the one hand of mobile terminals
which have a transmitter and receiver function, and on the other hand of
fixed access points which have a function of relaying the messages from
one cell to another. An ad hoc network, conversely, consists only of
mobile nodes and does not require the implementation of fixed
infrastructures to execute the functionality for routing the information
transmitted.

[0004] One of the problems encountered in implementing a system for
transmitting data within an ad hoc mobile network is that of the
effective sharing of the spectral resources with the aim of offering fast
access to the communication medium to all users, for high-throughput or
discreet communications.

[0005] The prior art describes several multiple access methods which allow
several terminals connected to the same transmission medium to share the
available passband without interfering with one another.

[0006] A first existing multiple access method is the Time Division
Multiple Access or TDMA method. This method allows all the users to
communicate on one and the same given frequency by allocating them time
segments during which they can transmit (respectively receive) data. This
method is notably used in the GSM ("Global System for Mobile
communications") European cellular telephony system. A drawback of the
TDMA multiple access method is that the access time to the medium for a
user is constrained to wait for a time segment which has been allocated
to this user. This transmission lag then brings about a potentially
appreciable latency time. A second drawback of this access method is that
a guard time must be introduced between each time segment to take account
of the signal propagation time, so as to prevent two distinct users from
interfering with one another. The guard time is dimensioned as a function
of the maximum distances between the users. The introduction of a guard
time then brings about a decrease in the available passband.

[0007] A second known multiple access method is the Frequency Division
Multiple Access or FDMA method. This method makes it possible to share a
frequency span among several users by allocating each user one or more
frequency bands in which he is authorized to communicate. A drawback of
the FDMA multiple access method is that a guard band must be introduced
between each frequency band so as to prevent two distinct users from
interfering with one another. The size of the guard bands is dimensioned
as a function of the performance of the bandpass filters and of the
maximum relative speed between users (Doppler effect). The introduction
of guard bands also brings about a decrease in the available passband.

[0008] A third multiple access method is the Code Division Multiple Access
or CDMA method. This method is based on the known direct-sequence spread
spectrum techniques. This method allows all the users to communicate on
one and the same given frequency by allocating them orthogonal codes used
to spread and despread the useful signal. However, the CDMA access method
exhibits a limitation within the framework of a mobile ad hoc network. It
is sensitive to the known problem of the "near-far effect". This problem
occurs when a terminal transmits with high power towards a far distant
terminal, this signal then saturating the nearer terminals.

[0010] Another problem raised when one seeks to deploy a mobile ad hoc
network is the specification of the type of antennas used. In order to
guarantee a link with very high throughput, that is to say of the order
of 100 Mbits/s, between two terminals of the network, and to do so with a
maximum range, it is preferable to use directional antennas since they
exhibit a more appreciable directivity gain in a given direction, thus
making it possible to concentrate the whole of the transmission power
towards the receiver terminal.

[0011] Finally, in a mobile network, the nodes move in the course of time,
thus posing the problem of the dynamic reallocation of the radio
resources by the access method. The method must notably be capable of
taking account, as effectively as possible, of frequent changes of
topology so as to cover the communication requirements of the nodes.

[0012] The present invention proposes notably a multiple access method
tailored to the effective sharing of resources within a mobile ad hoc
network. Each platform is allocated a frequency band that it manages
through a CDMA access method so as to simultaneously receive the signals
arising from other platforms. When transmitting, each platform uses a
code allocated by the addressee to transmit in the latter's frequency
band.

[0013] The proposed access method differs from the TDMA or TDD ("Time
Division Duplexing") access methods in the sense that it does not
introduce any latency time related to waiting for a radio resource in
order to transmit.

[0014] The access method according to the invention makes it possible to
limit the drawbacks related to several prior art access methods. [0015]
The use of spreading codes according to a CDMA method makes it possible
to limit the loss due to the guard bands required for a prior art FDMA
method. Indeed, the method according to the invention makes it possible
to multiplex several links within each band thus limiting the number of
necessary bands. [0016] The use of distinct reception frequency bands
allocated to each node of the network makes it possible to limit the
problem of the "near-far effect" introduced by the CDMA access methods.
Indeed, by separating the transmissions so that in a given band it has
only a single receiver, the problem of the "near-far effect" is avoided
by design. The proposed access method also implements mechanisms for
spatial reuse of the frequency bands.

[0017] The invention applies notably within the framework of the following
limitations: [0018] The number of available frequency bands is greater
than or equal to the number of network participants. [0019] There is no
spatial reuse of the frequency bands.

[0020] The invention also makes it possible to lift these limitations and
proposes that the potential number of users of the network be increased
by reusing the frequency bands on several platforms of the network. So
that the sharing of these bands is as effective as possible, an algorithm
for dynamically allocating the frequency bands is used with the aim of
minimizing the interferences between each platform reusing one and the
same frequency band. The allocation of the frequency bands is done on the
basis of an established network constructed beforehand by a network
construction algorithm. In this way, two nodes which possess a
transmission link between them are allotted different reception frequency
bands. In the case of the reuse of a frequency band, a check is carried
out so that the disturbance level induced by one link on another remains
below a predefined threshold. This check thus also eliminates the problem
of the "near-far effect".

[0021] The reuse of the frequency bands in a mobile network involves
frequent changes of band. These changes must be performed without any
dead time so as to retain the property of being able to transmit without
appreciable latency at any instant. A synchronous frequency changing
mechanism, also known by the term "handover", is proposed.

[0022] The invention is advantageously applicable in respect of
directional antennas which make it possible to obtain appreciable
throughputs and ranges. The proposed method exploits the directivity of
the antennas to increase the spatial reuse of the frequency bands and
thus increase the total throughput in the ad hoc network.

[0023] The subject of the invention is a method for multiple access to the
radio resources in a mobile ad hoc network comprising a plurality of
communicating mobile terminals or nodes having known geographical
coordinates, characterized in that it comprises at least the following
steps: [0024] a step of constructing the said network making it
possible to generate a plurality of active links between at least two
active nodes of the said network, an active link being composed of two
pairs of channels, assigned respectively to transmission and to
reception, each comprising a signaling sub-channel and a communication
sub-channel, a link being activated when the degree of relevance of the
said link is greater than a given constant, the degree of relevance of
the said link being determined at least as a function of criteria
regarding distance between the said nodes and/or of criteria regarding
priority between the said nodes, each of the said active nodes of the
said network comprising a neighborhood table which groups together the
set of active nodes of the said network to which it is directly linked by
an active link, [0025] a step of allocating reception frequency bands to
each of the said active nodes of the said network, the said frequency
bands being of identical width and spaced apart by a given minimum guard
band, the said allocating being performed in the following manner:
[0026] if the number of active nodes of the said network is less than or
equal to the number of frequency bands available, a different reception
frequency band is allocated to each of the said nodes, [0027] if the
number of active nodes of the said network is strictly greater than the
number of frequency bands available, a reuse of the said frequency bands
is effected in such a way that the allocation is performed so as to
minimize the interferences between two active nodes which have the same
reception frequency band, [0028] a spreading code allocation step
performed by each of the said receiver active nodes of the said network
for each of the said neighbor transmitter nodes to which it is directly
linked by an active link according to an asynchronous CDMA method, the
said neighbor transmitter nodes communicating with the said receiver node
in the reception frequency band which has been allocated to it, [0029] a
power control step performed by each of the said receiver nodes of the
said network for each of the said neighbor transmitter nodes to which it
is directly linked by an active link, the said power control step
implementing a different power regulation loop for each of the said
active links emanating from the said receiver node having the function of
ensuring that all the signals received by the said receiver node
originating from the said transmitter nodes are so with the same power
level.

[0030] In a variant embodiment of the invention, the same reception
frequency band is allocated to a plurality of distinct active nodes if
and only if the disturbance level induced by an active link of one of the
said nodes on an active link of another of the said nodes is below a
given threshold.

[0031] In a variant embodiment of the invention, a change of reception
frequency band of a node of the said network is performed during
transmission and this change is announced to the other nodes of the said
network with the aid of announcement messages containing at least the
instant of the change and the new frequency band.

[0032] In a variant embodiment of the invention, each of the said nodes
keeps up to date a geographical database comprising its geographical
coordinates as well as that of the nodes of its neighborhood table, the
said geographical coordinates being provided by a mechanism external to
the said network such as a long-range omnidirectional waveform, a
tactical data link or a radar system.

[0033] In a variant embodiment of the invention, each of the said nodes
keeps up to date a geographical database comprising its geographical
coordinates as well as that of the nodes of its neighborhood table, the
said base being constructed and maintained with the aid of signaling
messages dedicated for this purpose and transmitted between the nodes of
the said network.

[0034] In a variant embodiment of the invention, access to the radio
resources of a new node entering the said network is carried out with the
aid at least of the following steps: [0035] the said entering node
randomly selects a reception frequency band from among those available,
[0036] the said entering node transmits at least one network entry
message in each of the frequency bands available with a transmit power
whose level increases progressively until a response is obtained, the
said entry message containing at least the geographical position of the
said entering node, its speed, its acceleration and its reception
frequency band, [0037] on receipt of a network entry message, an
addressee node updates at least its geographical database and its
neighborhood table and responds through a response message transmitted in
the reception frequency band contained in the said network entry message
with a transmit power calculated at least as a function of the distance
between the said entering node and the said addressee node, the said
response message containing at least the geographical position of the
said addressee node, its speed, its acceleration and its reception
frequency band, [0038] on receipt of a response message, the said node
entering the network updates at least its geographical database and its
neighborhood table.

[0039] In a variant embodiment of the invention, the dispatching of the
said network entry and response messages is done under access contention,
using a single spreading code known to all the nodes of the network.

[0040] The subject of the invention is also a communication system onboard
a mobile node communicating within a mobile ad hoc network, characterized
in that it comprises at least one three-dimensional geographical database
and a processor suitable for implementing the steps of the above-defined
method for multiple access to radio resources.

[0041] The subject of the invention is also a mobile ad hoc network
comprising a plurality of mobile nodes, characterized in that the said
nodes carry onboard such a communication system for multiple access to
the radio resources within the said network.

[0042] In a variant embodiment of the invention, the said mobile nodes are
aircraft.

[0043] Other characteristics will become apparent on reading the detailed
description given by way of nonlimiting example which follows, offered in
relation to appended drawings which represent:

[0044] FIG. 1, a diagram illustrating the general principle of the access
method according to the invention,

[0045] FIG. 2, a functional schematic of a system comprising two nodes and
implementing the access method according to the invention,

[0046]FIG. 3, a diagram illustrating the connectivity between several
nodes of a network for which the invention applies,

[0047]FIG. 4, an exemplary meshed network implementing the method
according to the invention,

[0048] FIGS. 5 and 6, two diagrams illustrating the management of a
dynamic frequency allocation change,

[0049] FIG. 7, a diagram illustrating the power control implemented with
two different types of antennas.

[0050] FIG. 1 illustrates on a time/frequency chart the principle of the
multiple access method according to the invention. The invention uses a
given number of identical width frequency bands B0, B1, B2, Bn spaced
apart at the minimum by a guard band δB 101,102 of determined width
so as to ensure an acceptable inter-band interference level, that is to
say below a given interference threshold. Inside each of these bands B0,
B1, B2, Bn, the communication is organized dynamically using an
asynchronous code division multiple access (CDMA) method. The codes used,
for example codes of Gold or Kasami type known to those skilled in the
art, are constructed on the basis of pseudo-random sequences and possess
a good mutual orthogonality coefficient whatever the temporal offset
between the various transmissions.

[0051] Each node of the network is allotted one of these frequency bands
B0, B1, B2, Bn that it subsequently uses solely for reception. It is this
receiver node which subsequently manages access to the radio resources of
all the other nodes which communicate with it on the allocated frequency.
Notably this receiver node manages the allocation of codes and
throughputs for the nodes which transmit to it, and it also manages the
power control loops.

[0052] The frequency bands B0, B1, B2, Bn, may be reused by several nodes
under the following conditions: [0053] The codes used by these nodes
possess a good orthogonality coefficient, [0054] The power level of a
code received by a node which is not the addressee of the communication
must not exceed a given threshold.

[0055] The duplexing used is frequency-only duplexing. For a given node,
transmission is possible only outside of the frequency band allocated for
reception. This gives rise to constraints on the possibilities of direct
communication. Moreover, in the case of frequency reuse, several nodes
possessing the same reception band may be prompted to establish a link
between themselves. Two solutions are then possible. The first consists
in establishing an indirect communication between these nodes by way of a
routing via third-party nodes. The second consists in establishing a
direct communication by reallocating new frequency bands on the basis of
the links of the established network.

[0056] FIG. 2 depicts functionally the elements necessary for the
implementation of the access method according to the invention.

[0057] A first mobile node 210a of the network, which may be an aircraft
in the case of an aeronautical network or any other mobile terminal
capable of communicating with a third party, communicates with a second
mobile node 210b through a traffic link 230 whose function is the
exchanging of useful data and a signaling link 220 whose function is the
exchanging of messages allowing overall management of the network. Each
of these nodes 210a, 210b comprises a processor which executes,
continuously, a method 201a, 201b for constructing the network according
to the invention which provides information on the state of the network
to a method 202a, 202b for allocating frequencies according to the
invention. These two methods are continually fed by a three-dimensional
geographical database 203a, 203b which updates and delivers,
continuously, the geographical position, the speed vector and
acceleration vector of the set of neighbor nodes. The knowledge of these
temporal spatial parameters at an instant t makes it possible to estimate
the position of the neighbor nodes at an instant t'>t. The 3D
geographical database 203a, 203b is fed periodically with a given minimum
period. This feed may be performed in various ways. Signaling messages
220 may be dedicated for this purpose to thus support the mechanisms for
managing the base 203a, 203b in an autonomous manner. The geographical
coordinates of the nodes can also be viewed as input data provided by
external mechanisms such as a long-range omnidirectional link, a tactical
data link or else the radar system of an airplane.

[0058]FIG. 3 illustrates the network construction mechanism implementing
the access method according to the invention. Each node Ni, Nj, Nk, Nl
executes a distributed network construction algorithm. This algorithm is
responsible for the dynamic management of the communication links between
the various nodes of the system. A link between two nodes is always
bidirectional and composed of four channels. In each direction
(transmit/receive), a traffic channel 302, 304 and a signaling channel
301,303 are used. For each of these channels 301, 302, 303, 304 a
spreading code or code based on a PN sequence and a power level are
allocated. Any node of the system Ni, Nj, Nk, Nl, has the possibility of
establishing a certain number n of communication links, the maximum
number of links being specific to each node and being determined as a
function of its antennal capabilities and of its reception capabilities
in terms of number of codes processed in parallel. The set of links
established with the aid of the network construction algorithm, at an
instant t, defines the topology of the network. Any node Ni communicates
with a subset of nodes Nj, Nk, Nl of the system via its n active links.
Optionally, intermediate nodes can ensure the routing function. [0059]
The network construction algorithm builds a graph covering all the nodes
of the system in radio range. It executes continuously, the connectivity,
that is to say the active links between nodes, changing continually. Any
link between two nodes of the network whose degree of relevance is
greater than a given constant is established. The degree of relevance is
calculated as a function of various dynamic and/or static criteria, local
and/or received. The various criteria used may be weighted to compute the
final calculation of the degree of relevance. The criteria used can also
vary from one type of platform to another. The criteria used are for
example, but not solely, geographical criteria such as the proximity of
two network nodes between which a link may be established. The shorter a
link, the more relevant it will be. [0060] A link between two nodes may
be defined as compulsory for reasons specific to the system. In this case
this link will have a maximum degree of relevance as soon as the nodes
are in radio range of one another.

[0061] Once the network has been constructed, each node Ni of the system
executes a distributed algorithm according to the invention for
dynamically allocating the reception frequency bands. The method consists
in allocating in a distributed manner the reception frequency bands to
the various nodes of the system with the objective of minimizing the
interferences related to the reassigning of the frequency bands to
various nodes. The method takes account of the mobility/restructuring of
the network by dynamically reallocating frequencies with the aim of
maintaining the established links.

[0062] Two typical cases are distinguished. The first case corresponds to
a scenario for which the number of nodes constituting the network is less
than the number of available frequency bands. In this case, the frequency
allocation does not require any reuse of certain frequency bands, the
solution is optimal and may be executed via a distributed algorithm or
simply by static configuration.

[0063] The second case requires a more elaborate solution. This is the
case where the number of nodes is now greater than the number of
available frequency bands. In this case the aim of the frequency
allocation is to minimize the interferences between two nodes which
possess the same reception frequency band. A possible method for this
purpose consists in reducing to the known problem in graph theory of the
k-coloring of a planar graph. This method consists in assigning a
frequency band to each node with the constraint that two neighbor nodes
do not have the same frequency band.

[0064] An example of network construction and allocation of frequency
bands for reception is illustrated in FIG. 4. Three frequency bands B1,
B2, B3 are available and must be allocated to 9 distinct nodes which
possess mutually active links defined by a network construction
mechanism. [0065] Node 701 receives data from node 702 in frequency band
B1. [0066] Node 702 receives data from nodes 701 and 704 in band B3.
[0067] Node 703 receives data from node 705 in band B3. [0068] Node 704
receives data from nodes 702,705 and 708 in band B2. [0069] Node 705
receives data from nodes 703,704 and 709 in band B1. [0070] Node 706
receives data from node 709 in band B2. [0071] Node 707 receives data
from node 709 in band B2. [0072] Node 708 receives data from node 704 in
band B3. [0073] Node 709 receives data from nodes 705,706 and 707 in band
B3. Two neighbor nodes of the network do not have the same reception
frequency band so as to limit the level of disturbance. One node is a
neighbor of another within the network if they are directly linked by an
active link.

[0074] The network construction algorithm and frequency band dynamic
allocation algorithm define respectively changes, also known in the art
as "handovers", of links between nodes and changes of reception frequency
band. The changes of links between nodes can occur, for example, when two
nodes which have an established link move too far apart, the network
construction mechanism then regenerates new links. The changes of
reception frequency band occur, for example, when two nodes which
communicate reception-wise on the same frequency band approach one
another and this may create too appreciable an interference level.

[0075] When a change is decided by a given node, this node propagates
announcement messages, via its signaling channel, to all the pertinent
nodes of the system. The announcement message contains, for example, the
date at which the change must take place as well as information such as a
new frequency band, a power level or a PN code. FIG. 5 illustrates the
dispatching of announcement messages 401, 402, 403 by a node Ni to its
neighbors Nj, Nk, Nl announcing the future change 410 of link or of
frequency.

[0076] The transmitter nodes must ensure that the transmission of a data
packet does not occur during a change of frequency band, such as
illustrated in the left part of FIG. 6. A transmission is in progress in
the frequency band Bi-1 for which the data packets 501,502 have
already been transmitted, the packet 503 cannot be undergoing
transmission when a change 504 from band Bi-1 to Bi occurs. The
right part of FIG. 6 shows that the transmitter node must wait for the
change of frequency band 504 of the receiver node before transmitting the
data packet 503.

[0077] In order not to introduce any latency in the communication, it is
possible, in a variant embodiment of the invention, for each node to
possess the capacity to receive on two bands Bi-1, Bi simultaneously
during the period of the change. The old link is then broken only when
the new one is fully operational.

[0078] In mobile communication systems based on a network comprising an
infrastructure and implementing a code division multiple access (CDMA)
method, such as for example UMTS systems, each node of the network
transmits on a single channel only, called the up channel, to a single
destination which is called the base station. The latter is tasked with
performing the relaying of the messages to more distant nodes. In such a
network, each node implements only a single power regulation loop.

[0079] In the system according to the invention, conversely, each node of
the system can communicate with several addressee nodes, the latter
organizing access to the radio resources by code division (CDMA) for
reception in different frequency bands and at different distances from
the transmitter node. Consequently, any node of the system implements n
power regulation loops, n being the number of direct active links which
link this node to its neighbors. Indeed, all the signals having one and
the same node as destination must be received at a power level closest to
the level required, any signal being added to the interferences undergone
by the other signals. The function of the power regulation loops
implemented by a node is therefore to adapt the transmit powers as a
function of the link budget of the active links which connect it to the
addressee nodes. The link budget depends notably on the relative
distances between each node. The power control loop is all the more
reactive the bigger the relative speeds of the nodes so as to remain
compatible with a highly mobile network.

[0080] By way of example FIG. 7 illustrates a control loop implemented
between two nodes 601 and 602 in a frequency band for reception equal to
B1. It also shows the interferences induced by node 601 on node 603 which
also uses the frequency band B1 for reception in the same way as node
602. Nodes 602 and 603 are situated at equal distances from the
transmitter node 601. The left part of FIG. 7 shows the case where a
directional antenna is used by the transmitter node 601, in this case the
transmitted signal 605 destined for node 602 is more weakly received 604
by the neighbor node 603. On the other hand in the case where an
omnidirectional antenna is used by the transmitter node 601, as is
illustrated in the right part of FIG. 7, the transmitted signal 606 is
received with the same amplitude by the addressee node 602 and the
neighbor node 603.

[0081] A particular case of the invention occurs in the case where the 3D
geographical database is updated by way of signaling messages and when a
mobile terminal makes its entry into the network which has been
previously established. A network entry protocol must therefore be
implemented. This protocol is based on the use of network entry messages.
These messages are dispatched under access contention, by using a unique
code known and decoded by all. The terminal entering the network randomly
selects a reception frequency band, from among those available, that it
will be able to alter over time until at least one response originating
from another node is obtained. It transmits network entry messages in
each of the frequency bands of the system so as to have a chance of
reaching at least one neighbor node. For the nodes using directional
antennas, the dispatching of network entry messages is done successively
in various sectors of space so as to obtain omnidirectional coverage. In
order not to dazzle any near nodes that might be receiving on the
frequency band of a network entry message, these messages are dispatched
by firstly using a reduced power which keeps increasing until a response
is obtained. Any network entry message contains at the minimum the
geographical position of the transmitter node, its speed vector and
acceleration vector as well as its reception frequency band. On receipt
of a network entry message, the addressee node updates its data
structures, including its 3D geographical database and its neighborhood
table.

[0082] Any node having received a network entry message responds through a
response message which is transmitted in the frequency band defined in
the network entry message. The response message transmit power is
calculated as a function of the distance of the source node and addressee
node and of the power level required for the reception of this type of
message. The distance between the two nodes is calculated as a function
of the geographical information of the node transmitting the response
message and of the information received in the input network entry
message. This information is extracted from the 3D geographical database.
The response message contains the geographical positions, the speed
vector and acceleration vector as well as the reception frequency band of
the nodes which are known to the transmitter node. On receipt of the
response message, the addressee node updates its various data structures,
including its 3D geographical database and its neighborhood table.

[0083] The invention has the advantage, by virtue of the proposed multiple
access method, of allowing optimal allocation of resources in terms of
latency times and of available passband between the various users of a
mobile ad hoc network. The latency of a point-to-point transmission
between two nodes of the network is reduced since access to the spectral
resource by a user is not constrained over time in contradistinction to
time division multiple access techniques.

[0084] Moreover the invention also presents the advantage of coupling two
functions, namely a multiple access method and a discretion method by
using the same mechanism, thereby making it possible to redistribute the
capacity which is unused in the case of discretion, that is to say of
reduced power, to the other users. Indeed, the signal transmitted by a
terminal may be rendered discreet by decreasing the transmit power and by
increasing the spreading factor which is a parameter of the CDMA access
method. By way of example, a spreading factor equal to 10 entails the
coding of a useful bit by 10 chips (or "slots") of the spreading code
used. The total energy radiated is distributed over the useful frequency
band and the power density per Hertz of the transmitted signal is 10
times lower before the despreading operation on reception. The spread
signal is therefore more difficult for a third party to detect. Each
platform can decide its degree of discretion independently of the others,
the drop in capacity brought about by an increased requirement for
discretion being redistributed to the other users sharing the same
frequency band.

[0085] The invention also makes it possible to dynamically manage the
allocation of the frequency bands allocated to reception for each node of
the network as a function of the spatial evolution of this network. In
particular when two terminals operating at the same reception frequency
approach one another, a reallocation of the frequency plan is
implemented.